This invention relates to a fuel assembly for light water type nuclear reactors, and more particularly to a fuel assembly using the uranium recovered from spent fuel.
In the conventional light water type nuclear reactor, for example, a boiling water type nuclear reactor, fuel assemblies containing enriched uranium having an average U.sup.235 enrichment of about 3% by weight are usually burnt up to a degree of burnup of about 30 GWd/mt and discharged from the nuclear reactor.
An example of the fuel assemblies is shown in FIG. 1. A fuel assembly 3 is constituted of a plurality of fuel rods 1 and a channel box 2. Since a control rod 4 or a neutron detector instrumentation pipe 5 is provided outside the channel box 2, the distance between the adjacent fuel assemblies 3 are so wide that the devices such as control rod 4, etc. can be inserted therebetween, and the spaces between the fuel assemblies are filled with cooling water. The upper and lower ends of fuel rods 1 are supported by upper and lower tie plates (not shown in the drawing), respectively. Channel box 2 is fixed to the upper tie plate and surrounds a bundle of fuel rods 1. Numeral 6 is water rod, and numeral 7 is a coolant region among the fuel rods and a coolant passes through the coolant region.
The following main phenomena appear in the core with the progress of burning:
(i) Consumption of fissionable materials (U.sup.235, Pu.sup.239, Pu.sup.241) PA1 (ii) Conversion of fertile materials (U.sup.238, Pu.sub.240) to fissionable materials PA1 (iii) Accumulation of fission products
One example of changes in the U.sup.235 enrichment (% by weight) with burning is shown in FIG. 2. As is obvious from FIG. 2, spent fuel still contains about 0.8% by weight of U.sup.235. For effective utilization of uranium source, it may be possible to reprocess spent fuel, supply the recovered uranium again to an enrichment plant and reuse the recovered uranium reenriched to the necessary enrichment. However, reenrichment of recovered uranium and reuse of it as UO.sub.2 fuel have the following problems.
With the progress of burning in a nuclear reactor, U.sup.235 is consumed by nuclear fission, as given in the above (i), while some of U.sup.235 is converted to U.sup.236 through absorption of thermal neutrons. U.sup.236 further absorbs the neutrons, producing Np.sup.237 through successive .beta.-decay. One example of changes in concentrations of U.sup.236 and Np.sup.237 (% by weight) with the progress of burning are shown in FIG. 3, where the curve 8 shows U.sup.236 and the curve 9 shows Np.sup.237, from which it is obvious that the spent fuel contains about 0.4% by weight of U.sup.236 and about 0.03% by weight of Np.sup.237. Both U.sup.236 and Np.sup.237 are neutron absorbers and their absorption cross-section for thermal neutrons at 2200 m/sec. is as large as 5.2 barns for U.sup.236 and 170 barns for Np.sup.237, as compared with 2.7 barns for U.sup.238.
When the recovered uranium obtained by separating Np.sup.237 from uranium through reprocessing is reused in UO.sub.2 fuel, reactivity lowering (which will be hereinafter referred to as "reactivity penalty") corresponding to the U.sub.236 concentration of UO.sub.2 fuel takes place. To overcome the reactivity penalty, the U.sup.235 enrichment must be increased, and this is quite contrary to the effective utilization of uranium source [T. Mikami: Kakurenryo Cycle no System (System for nuclear fuel cycle), published by Kyoritsu Publishing Co., Tokyo, March 1, 1980, pages 48-51].
The U.sup.236 concentration of UO.sub.2 fuel depends on a feed ratio of recovered uranium to natural uranium in an enriching plant. When a larger amount of recovered uranium is used to reduce the necessary amount of natural uranium, i.e. requirements for natural uranium, total absolute amount of U.sup.236 contained in UO.sub.2 fuel is increased, and thus the reactivity penalty due to U.sup.236 is increased. Thus is a disadvantage so far encountered in the reuse of recovered uranium.